1. Field of the Invention
The present invention relates to a current path cut-off mechanism in a cell and more particularly to a current path cut-off mechanism in a cell which is to be used for ensuring safety in the event of an increase in internal pressure of a cell case.
2. Description of the Prior Art
Heretofore, for example, lithium ion secondary cells capable of being charged and having a cell case with an organic solvent-based electrolyte sealed therein have been widely used as power supplies in portable devices such as portable telephones and personal computers.
In re-charging such a cell, if overcharging is made or if charging is performed with an electric current larger than a predetermined current, a problem will occur in the cell and a gas will be generated within the cell case, with consequent increase in internal pressure and temperature of the cell case, expansion and cracking of the cell case, and oozing out of the internal electrolyte to the exterior. Such a state may exert an adverse effect on the device which incorporates the cell.
Even if such a result does not occur, if the use of the cell is continued in the above abnormal condition, the expansion of the cell will become more and more conspicuous and may lead to bursting of the cell case. Therefore, it has so far been necessary to immediately stop the use of a cell which is found to be abnormal.
A conventional current path cut-off mechanism for preventing such bursting of a cell will now be described by way of such a circular cell as shown in FIGS. 7 and 8. A cell lid 3 is secured to a cell case 1 through a gasket 2 so as to seal the interior of the cell case 1 by caulking at its peripheral portion or by welding. Below the cell lid 3 is disposed an actuator 4 which can be displaced upward.
Vent openings 3a are formed in the cell lid 3, and upon upward displacement of the actuator 4 located below the cell lid, causing cleavage, the air present in the space between the cell lid 3 and the actuator 4 escapes to the exterior through the vent openings 3a.
The actuator 4 has an annular safety valve portion 4a which is formed in a generally horizontal flat shape by drawing with a press for example. The safety vale portion 4a is centrally formed with a protuberance 4b which projects downward, and radial grooves 4c are formed in the upper surface of the flat portion around the protuberance 4b. The protuberance 4b can be made uniform in height with little variations because it formed with a press or the like.
Below the actuator 4 is disposed a stripper 5 formed by molding a resin material for example. The stripper 5 is formed with a through hole 5a for insertion therein of the protuberance 4b of the safety valve portion 4a and also formed with a vent hole 5b as a through hole.
Below the stripper 5 is disposed an insulating member 6 formed by molding a resin material for example. The insulating member 6 is formed with a hole 6a which is in communication with the hole 5a of the stripper 5 and a vent hole 6b which is in communication with the vent hole 5b of the stripper 5.
A lead 7, which is a thin metallic plate, is attached to the back side of the insulating member 6, the protuberance 4b of the safety valve portion 4a is inserted into the holes 5a and 6a of the stripper 5 and the insulating member 6, respectively, and a bonding portion 7a of the lead 7 is bonded to the tip of the protuberance 4b by spot welding for example. At the right-hand end of the lead 7 is formed a connecting portion 7b which extends downward.
The actuator 4 and the lead 7 are in electric conduction with each other and the connecting portion 7b formed at the opposite end of the lead 7 is connected to a power generating element 8, with a current path being formed between the power generating element 8 and the cell lid 3.
Upon occurrence of a trouble in the interior of the cell and with consequent rise in internal pressure of the cell case 1, the gas whose pressure has increased flows from the vent holes 5b and 6b like arrow A in FIG. 8 and acts to push up the back side of the safety valve portion 4a.
This working force exerted on the safety valve portion 4a induces a concentrated stress on the bonding portion 7a of the lead 7. When this concentrated stress becomes larger than the shear stress of the bonding portion 7a, the bonding portion 7a is ruptured or stripped from the protuberance 4b, so that the safety valve portion 4a is displaced upward to cut off the electric connection between the lead 7 and the actuator 4, whereby the current path of the cell is cut off.
As a result, the current flow in the interior of the cell is cut off to prevent a rise in internal pressure of the cell case 1 and thus the burst of the cell can be prevented.
In the above conventional current path cut-off mechanism, since the stripper 5 and the insulating member 6 are formed by molding, variations in plate thickness are large due to variations in molding conditions. If the variations in plate thickness of the stripper 5 and the insulating member 6 are large, there arises the problem that variations in gap size between the actuator 4 and the lead 7 also become large.
More particularly, if the plate thickness of the stripper 5 and that of the insulating member 6 both vary on the minus side, the protuberance 4b will strike against the lead 7 at the time of bonding the actuator 4 to the lead 7. This results in the safety valve portion 4a being somewhat displaced upward to form a slight gap between the stripper 5 and the underside of the safety valve portion 4a, as shown in FIG. 9.
With the safety valve portion 4a somewhat displaced upward, the working force for displacing the safety valve portion varies. Thus, this sometimes results in unreliable operation of the safety valve portion 4a even if the internal pressure of the cell should increase above a predetermined value.
On the other hand, when the plate thickness of the stripper and that of the insulating member 6 both vary on the plus side, as shown in FIG. 10, a gap is formed between the tip of the protuberance 4b and the lead 7 and it is therefore impossible to bond the protuberance to the bonding portion 7a. Thus, the results is that sometimes the foregoing current path cannot be formed. Even if the protuberance 4b is bonded forcibly to the bonding portion 7a, the bonding strength of the bonded portion will become lower than a desired value, thus giving rise to the problem that the safety valve portion 4a operates at an internal pressure of the cell below a predetermined value.
A problem also exists when in assembling a conventional current path cut-off mechanism in a cell, the lead 7 is stripped from the insulating member 6 or is deformed due to mishandling of the connecting portion 7b extended downward from the lead 7, thus giving rise to variations in bonding strength of the bonded portion between the protuberance 4b and the bonding portion 7a.